Poly(tetramethylene ether) glycol (PTMEG) is a commodity in the chemical industry, widely used in the manufacture of block copolymers in combination with polyfunctional urethanes and polyesters such as polybutylene terephthalate. It is commonly prepared by reaction of tetrahydrofuran (THF) with a strong acid catalyst such as fluorosulfonic acid and then quenching and hydrolyzing the product with water.
While this process has proved to be quite satisfactory, it is not as efficient as desired because the acid catalyst cannot be recovered and reused again. Moreover, disposal of the spent acid is a problem because of its toxicity and corrosiveness.
U.S. Pat. No. 4,120,903 describes the preparation of PTMEG by the polymerization of THF using, as catalysts, perfluorinated ion-exchange polymers with pendant sulfonic acid groups (PFIEP-SO.sub.3 H) which have a low enough solubility to allow easy separation from the product, thus permitting their reuse. A method for avoiding the disposal problem traditionally associated with the strong acid catalyst was therefore possible based on the reaction equations (1) through (4) of U.S. Pat. No. 4,120,903, Column 1, where P-SO.sub.3 H has the same meaning as PFIEP-SO.sub.3 H herein. The PFIEP-SO.sub.3 H catalyst is further described by F. J. WALLER and R. W. Van Scoyoc, CHEMTECH, Jul., 1987, pages 438-441.
There are two drawbacks to implementation of this technology, however, in that (1) the polymerization is sensitive to moisture necessitating that both the catalyst and the THF be specially dried before use and (2) the PTMEG made using this process had a number average molecular weight 10,000 or greater.
One approach to circumvent these drawbacks by Heinson et al using the PFIEP-SO.sub.3 H catalyst of U.S. Pat. No. 4,120,903 above is disclosed in U.S. Pat. No. 4,163,115. This patent discloses maintaining an anhydrous reaction environment by carrying out the polymerization of terahydrofuran in the presence of an acylium ion precursor , i.e. a carboxylic acid anhydride such as acetic anhydride and optionally the corresponding carboxylic acid. Esters of the PTMEG were formed with facile control of molecular weight in the desired commercial range or up to very high molecular weights. Recovery of the PTMEG, however, required saponification or methanolysis, and thereby diminishing the advantages of this process.
An insoluble catalyst, less sensitive to moisture in the polymerization system, and which could yield commercial grade PTMEG in the desired molecular weight ranges of 250 to 4,000 would have a distinct advantage.
The present invention is an improvement in the process for PTMEG manufacture within the commercially important molecular weight range of 250 to 4,000 in which THF containing the amount of water usually found in commercial polymerization-grade material, and an insoluble catalyst are brought together under conditions suitable for polymerization. After the desired polymerization has taken place, the catalyst may be separated from the bulk of the reaction mass for reuse and the PTMEG isolated by the usual methods. Part of the water introduced into the system either with the catalyst or the THF, acts as the chain terminator for the polymerization and, as such, is consumed. The catalyst of this invention is a blend of a perfluorinated ion exchange polymer containing pendant sulfonic acid groups (PFIEP-SO.sub.3 H) with a perfluorinated ion exchange polymer containing pendant carboxylic acid groups (PFIEP-CO.sub.2 H). The PFIEP-SO.sub.3 H/PFIEP-CO.sub.2 H may be used as is or it may be further blended with other, inert polymers before use. Ordinarily, these catalysts would not yield commercial grade PTMEG in the desired molecular weight range from 250 to 4,000 largely due to the poor access of the active sulfonic acid end groups to THF. The accessibility is considerably improved by swelling the catalyst in THF after activation by partial dehydration through (1) heating to 90.degree. C. or (2) by leaching the catalyst with dry THF or (3) drying the catalyst using a dehydrating agent like trimethyl orthoformate or (4) by distilling off the water contained in the catalyst or through any combination of the above methods.